In mobile communications, in order to realize high quality communication by efficiently using limited radio resources, various communication control technologies have been proposed. Most of such communication control technologies use control in accordance with the moving speed of the mobile terminal. For example, the parameters used for adaptive equalization to reduce the influence of signal interference contained in the received signal are determined in accordance with the moving speed of the mobile terminal. Further, in power control in which the transmit power varies in accordance with the reception conditions at the mobile terminal and in adaptive modulation in which the code modulation scheme is selected depending on the reception conditions at the mobile terminal, the moving speed of the mobile terminal is used to estimate the reception conditions. For this reason, precise estimation of the moving speed of the mobile terminal is needed in order to realize high-quality communication.
As the indicator representing the moving speed of the mobile terminal, the fading Doppler frequency is usually used. Because of reflection of signals by buildings and others, a plurality of communication signal propagation channels are formed between the base station and the mobile terminal. Signals that have passed through multiple propagation channels vary in amplitude and phase with time when they are superimposed and received at the mobile terminal or base station. This phenomenon is called fading. As a scale to measure the rate of time variation of the fading, the fading Doppler frequency may be used. Since time variation of fading arises due to movement of the mobile terminal, the moving speed of the mobile terminal corresponds to the fading Doppler frequency.
As a method for estimating the fading Doppler frequency, there is a known method which uses the time domain channel correlation value, or the normalized covariance of the channel estimated value that shows the channel characteristics of radio channels. In this method, the base station or mobile terminal, based on the received signal, acquires an estimate of the channel value at regular intervals, and estimates the time domain channel correlation value based on the estimated values of channel. Since it is known that the time domain channel correlation value and the fading Doppler frequency are related by a zero-ordered Bessel function, the base station or the mobile terminal, based on this relationship, estimates the fading Doppler frequency from the estimated time domain channel correlation value (see Patent Document 1).
Since it is possible to accurately estimate the moving speed by estimating the fading Doppler frequency with precision, appropriate communication control can be carried out. However, when the signal level of the received signal is low, the ratio of the noise component contained in the channel estimated value becomes large, so that the time domain channel correlation value tends to be estimated to be smaller than the actual value. As a result, a problem occurs in which the estimation accuracy of the fading Doppler frequency decreases.
To resolve this problem, a method has been proposed that prevents deterioration in the estimation accuracy, in which the Doppler frequency fades, by correcting the time domain channel correlation value based on the ratio of the power of the true signal component and the power of the noise component in the received signal, namely, the signal-to-noise ratio (Signal-to-Noise Ratio: which will be abbreviated hereinbelow as SNR)(see Non-patent Document 1).